A retrospective multicentre clinicopathological study was performed on sequential bone marrow trephine biopsies in 100 patients with Ph1+-chronic myelogenous leukaemia (CML) to elucidate the effect of interferon (IFN) alpha 2b and hydroxyurea (HU) treatment on myelofibrosis and megakaryopoiesis. According to strictly defined therapeutic regimens, 38 patients received IFN as monotherapy, 23 patients a combination of IFN and HU and 39 patients HU only. Using standardized intervals of biopsies and histochemical and morphometric methods, a significant increase in reticulin fibre density and in the number of CD61+ megakaryocytes was detectable in the majority of IFN-treated patients. To a lesser degree, these changes were also expressed in the cohort with a combined IFN and HU regimen. In contrast to these findings, in the group of patients with HU as single-agent treatment, a stable state or reversal of myelofibrosis was detectable together with corresponding changes in megakaryopoiesis. Further evaluations revealed that these effects had occurred within the first year, mostly after 6 months of treatment, and were prominently expressed in those patients with a slight to relevant grade of myelofibrosis at presentation. In conclusion, this study provides persuasive evidence that monotherapy by IFN exerts a fibrogenic effect, while HU treatment seems to prevent and even resolves bone marrow fibrosis in CML. Probably, in relation to the complex pathomechanisms responsible for the generation of myelofibrosis, the changing content of reticulin fibres was usually accompanied by corresponding alterations in the number of CD61+ megakaryocytes, including atypical microforms and precursor cells.